For expository convenience, the present invention will be illustrated with reference to one particular application thereof, namely that of sensing sparking noise from electrical power lines. However, it should be recognized that the invention is not so limited.
Electrical power lines carry a signal which is typically a periodic 60 Hz AC voltage. In an AC signal, the voltage potential varies as a sinusoid between positive and negative maximum values with periodic zero crossings.
At the zero crossings, the power line has a voltage potential near that of ground. At the maximum excursions of the signal, however, there may be enough of a potential difference between the power line and adjacent objects to induce a spark. Sparking is most likely to occur with high voltage power lines, or in settings where the physical integrity of the power line is compromised (i.e. loose hardware, etc.). However, sparking can occur wherever there is a power signal, and is thus a widespread phenomenon.
Sparks create wideband radio frequency (RF) signals that can interfere with all manner of radio and television equipment. However, the radio signals produced by such sparking also provide a means by which the sparking power line can be tracked down and identified. Electric utility companies often have crews of troubleshooters who utilize radio receivers and directional antennas to locate sparking power lines so they might be repaired. In this way, interference to radio and television equipment can be traced and eliminated.
A hindrance to such tracing of sparking power lines is that power lines are not the only sources of electrical sparks. Vehicle ignition systems (e.g. "spark" plugs) also produce spark noise that generates similar RF interference. Due to the alternating voltage of power signals, power line sparking generally occurs with a repetition rate of 60 Hz (120 Hz in instances where sparking occurs with equal regularity on the positive and negative excursions). Depending on engine speed, vehicle ignition systems can also generate sparks at 60 Hz and multiples thereof. RF interference from vehicle ignition systems is especially troublesome when attempting to trace a sparking power line while traveling in a motor vehicle.
There are some differences between periodic ignition sparks and power line sparks. Power line sparks typically occur more than once during a 60 Hz sinusoidal power signal cycle. Sparks may occur during the negative half as well as the positive half of the power signal cycle. Also, after a spark occurs, the potential difference is quickly recharged to a voltage where sparking may again occur. (The power line may be modelled as an R-C circuit, with a capacitance formed by an insulator, and then a large resistance--such as a wooden pole--to ground.) Since the recharging is quick, several sparks and recharges usually occur in succession during the same half cycle of the power signal. Thus, during each half cycle, a series of several closely spaced sparks typically occur.
In accordance with the present invention, a method and apparatus are provided for isolating specific noise components in an input signal so they can be processed in a desired manner. The method comprises determining an interval during which the noise component is expected to occur, and isolating a portion of the input signal occurring during that interval. Determination of the interval is made by sensing noise impulses in the input signal, temporally correlating the noise impulses, and estimating the time that the noise impulses are expected to recur. The input signal can then be passed during the interval to an output signal to enhance the noise component, or the input signal can be blanked from the output signal to reduce the noise component.
The preferred apparatus comprises a noise impulse detector, a memory, an electronic switch, and a general purpose processor programmed according to the method of the present invention. The noise impulse detector senses noise impulses in an input signal. The time that noise impulses are sensed is recorded by the processor in the memory. The processor also temporally correlates the sensed noise impulses to determine an interval during which a particular noise component of the input signal is expected to recur. The processor actuates the electronic switch during the interval to pass or blank the input signal during the interval.
The present invention has particular application to the detection of radio frequency interference or noise impulses caused by power line sparks. As described above, such noise impulses can be expected to occur at intervals correlated to a 60 Hz power signal cycle. Thus, a window of time during which a power line spark may be expected will occur approximately one 60 Hz cycle (16.7 milliseconds) after a first power line spark is detected. Therefore, the present invention can be practiced by receiving a radio frequency input signal containing wideband noise from power line sparks, detecting a noise impulse, predicting an interval for a next noise impulse (i.e. approximately 16.7 milliseconds after the detected noise impulse), and passing the input signal during the predicted interval. This particular application of the present invention effectively passes power line noise while blanking non-periodic noise. Of course, non-periodic noise which coincidently occurs during those intervals in which the input signal is passed will not be blanked.
The foregoing implementation of the invention, however, does not discriminate against other sources of 60 Hz noise. As described above, other sources of periodic 60 Hz noise impulses exist, and these sources would also be detected by the foregoing technique. Accordingly, the methodology may be further refined by exploiting a unique characteristic of power line sparks, such as their tendency to occur in closely spaced groups. In such a refined methodology, it is not enough that a second noise impulse be detected approximately 16.7 milliseconds after a first impulse. Instead, the second noise impulse must be detected approximately 16.7 milliseconds after a group of closely spaced first noise pulses. Thus implemented, the present invention enhances the radio frequency interference caused by power line sparks while blanking non-periodic noise and periodic noise from other sources.